1. Field of the Invention
The present invention relates to an optical system used for image pickup apparatuses such as digital still cameras and video cameras, and particularly to an optical system suitable for a case where an image restoration process is performed on an image produced by image capturing.
2. Description of the Related Art
Images obtained by capturing an object through optical apparatuses such as digital cameras and interchangeable lenses include a blur component as an image degradation component caused by various aberrations of an image capturing optical system (hereinafter simply referred to as “an optical system”), such as spherical aberration, coma aberration, field curvature and astigmatism. Such a blur component is generated because a light flux emitted from one point of an object forms an image with some divergence on an image pickup surface, the light flux being normally converged at one point if there is no aberration or diffraction.
The blur component herein is optically expressed as a point spread function (PSF), which is different from blur caused by defocusing. Moreover, color blur in a color image caused due to longitudinal chromatic aberration, chromatic spherical aberration or chromatic coma aberration of the optical system can be said to be a difference between blurring degrees of respective light wavelengths. In addition, horizontal color shift caused by chromatic aberration of magnification of the optical system can be said to be position shift or phase shift of color light components caused by differences of image capturing magnifications for the respective color light components.
An optical transfer function (OTF) obtained by performing Fourier transform on the point spread function (PSF) is frequency component information of aberration, which is expressed by complex number. An absolute value of the optical transfer function (OTF), that is, an amplitude component is called a modulation transfer function (MTF), and a phase component is called a phase transfer function (PTF). The MTF and PTF are respectively a frequency characteristic of the amplitude component and a frequency characteristic of the phase component of image degradation caused by the aberration. The phase component is herein shown as a phase angle by the following expression where Re(OTF) and Im(OTF) respectively represent a real part and an imaginary part of the OTF.PTF=tan−1(Im(OTF)/Re(OTF))
Thus, the optical transfer function (OTF) of the optical system degrades the amplitude component and the phase component of the image. Respective points of the object in the degraded image are asymmetrically blurred like coma aberration.
Moreover, the chromatic aberration of magnification is generated in an image pickup apparatus that captures, according to its spectral characteristics, color components such as R, G and B whose imaging positions are mutually shifted due to differences of imaging magnifications of its optical system for respective light wavelengths. Therefore, not only the shift of the imaging positions among the color components is generated, but also shift of imaging positions among wavelengths in each color component, that is, image spread due to the phase shift is generated. Thus, although, strictly speaking, the chromatic aberration of magnification is not merely a color shift that is parallel shift of colors, this specification treats the color shift as the chromatic aberration of magnification.
As a method for correcting such degradation of the amplitude component (MTF) and degradation of the phase component (PTF) in the degraded image (input image), there is known a method using information on the optical transfer function (OTF) of the optical system. This method is referred to as “image restoration” or “image recovery”, and a process for correcting the degraded image by using the information on the optical transfer function (OTF) of the optical system is hereinafter referred to as “an image restoration process” or simply “image restoration”. As one of the image restoration processes, a convolution method that performs convolution of an image restoration filter having an inverse characteristic to the optical transfer function on an input image in a real space (see Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-509333). Moreover, Japanese Patent Laid-Open No. 2010-56992 discloses a method of performing the image restoration process on an input image with filter coefficients held for correcting degradation of the input image.
On the other hand, it is generally difficult to decrease variation of aberration of an optical system due to variation of object distance. Therefore, emphasis on optical performance in a state where the optical system is focused on an infinite object distance is likely to deteriorate the aberration in a state where the optical system is focused on a closest object distance. Japanese Patent Laid-Open No. 2011-028166 discloses a method of providing a desired resolution (MTF width) by generating spherical aberration in order to perform good image restoration on an image obtained by image capturing of objects whose distances are mutually different.
When premising that the image restoration is performed, some aberration of an optical system is allowed, which enables downsizing of the optical system, higher magnification thereof and higher image quality over an entire object distance range. In other words, correcting, by using the image restoration image, degradation generated due to increase of refractive powers of respective lens units constituting the optical system in order to decrease its size, increase its magnification and improve image quality over the entire object distance range enables higher quality image while achieving downsizing of the optical system and higher magnification thereof.
However, an excessive strength of the image restoration emphasizes a noise component included in the degraded image. Moreover, an excessive allowable aberration amount for the optical system makes it impossible to sufficiently correct the image degradation by the image restoration. For example, an excessive field curvature causes tilt of an image plane on an image sensor even though there is a little manufacturing error of lenses constituting the optical system or a little tilt of the image sensor, which significantly causes an “uneven blur” which is asymmetry of resolving power. In this case, it is impossible to correct the image degradation well by the image restoration.
Therefore, in order to achieve the decreases in size of the optical system, the increase in magnification thereof and the improvement in image quality over the entire object distance range on the premise that the image restoration is performed, it is necessary to consider generation of appropriate aberration for the image restoration. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-509333 and Japanese Patent Laid-Open Nos. 2010-56992 and 2011-028166 do not disclose such appropriate aberration for the image restoration at all.